1. Field of the Invention
This invention relates to corrosion sensors, particularly those which measure instantaneous or cumulative corrosion rates.
2. Description of Related Art
There are several methods in the prior art of determining various corrosion rates. One method is called the weight-loss method, and it involves exposing accurately pre-weighed panels of materials to the particular environment whose corrosion rate is to be measured (e.g., the atmosphere or seawater). After being exposed for a lengthy period of time, usually ranging from one to twenty years, the panels are cleaned of any corrosive by-products which have accumulated and then re-weighed to determine the amount of actual panel material which has been lost as a result of the corrosion. This provides a very accurate measure of the total corrosive loss, which is called the interval weight loss. This measurement is then used to try to obtain an average corrosion rate (weight loss per unit time).
Based on the amount of the weight loss, the time of exposure and several other factors, such as the amount of panel area exposed and the panel material density, an average corrosion rate is computed for the entire period. Unfortunately, the average corrosion rate computed from the weight loss measurement is not necessarily accurate. This is because it assumes that the corrosion rate has been constant during the long period involved. This is usually not true. Indeed, tests have indicated that the long term corrosion rates of certain alloys vary significantly over time. For example, the corrosion rate of steel in seawater actually decreases as the time of exposure increases.
The average corrosive rate computation based upon the long term weight loss measurement is also inaccurate for another reason. It assumes the environment being measured remains unchanged over the long period, when in reality the environment may exhibit both short and long term changes that significantly effect the corrosion rate. As a result, a long term weight-loss measurement often cannot be depended on as a basis for accurately determining either a specific short term corosion rate or the instantaneous corrosion rate at any given time. Similiarly, a short term weight loss measurement would not necessarily produce accurate information about long term corrosion. Accordingly, real-time monitoring of the actual instantaneous corrosion rate, along with a means to assess cumulative corrosion damage would be more useful.
The measurement of the instantaneous corrosion rate poses some problems. For example, in terms of atmospheric corrosion, that rate depends upon a number of factors, one of the most important of which is called time of wetness. Time of wetness sensors do exist in the prior art. They are called built-up sensors because they consist of a stack of metal plates separated from each other by an insulator, with alternate plates being electrically shunted. When the surfaces of the plates are wet, a small voltage is applied to them, and this results in a current flow between the plates. That current flow is proportional to the rate of corrosion actually occuring on the plates acting as the anodes. This generally indicates the severity of the corrosion occuring at the time. As a result, the instantaneous corrosion rate can be determined from this current by what is called a linear polarization resistance method ("LPR"). For small potential differences between the plates of the order of 10 mV or less, the resulting current will be proportional, based on what is called the polarization resistance. This resistance is inversely proportional to the natural corrosion rate of the plates. Accordingly, the current may be measured by means of a zero resistance ammeter and then converted to an instantaneous corrosion rate by means of Faraday's law.
There are several drawbacks to this. First, the instantaneous corrosion rate itself may vary by several orders of magnitude over a short time, and thus, the computed rate may or may not by itself produce much meaningful information. Secondly, these sensors are all hand made. This is not only time-consuming and expensive, but it also means that each sensor must be individually calibrated to allow for the inherent variations between the hand-made devices. Consequently, while this type of sensor is well suited for measuring short term corrosion or very low corrosion rates, it is not in widespread use because of its size (as a result of the stacked plates) and its cost.
Although the instantaneous corrosion rate is a useful measurement in some respects, another at least complimentary if not more meaningful corrosion rate is called the time averaged corrosion rate. It represents how much metal has been lost over a limited period of time. Generally, the time averaged rate of corrosion is determined by a method called the electrical resistance ("ER") method. In that method, the sensor comprises a long strip of metal which is exposed to the environment being measured. The corrosion will reduce the dimensions of the exposed strip, and as its thickness decreases, its electrical resistance will increase. This increase in the resistance can be converted into a time averaged corrosion rate, which is also a cumulative corrosion measurement.
The drawback of this ER type of sensor is its considerable bulk which is due to the long length of the exposed strip. Because of the small resistance involved, the strip must be long in order to make the initial resistance measureable. The alternative is to reduce the thickness of the strip. While such a reduction increases the resistance, it also shortens sensor life, as the strip will corrode entirely through in a shorter period of time.
As a result of all of this, there is a need for a corrosion sensor which is small, inexpensive to manufacture and is preferably capable of measuring the instantaneous corrosion rate as well as the time averaged corrosion rate.